The present invention relates to a gasket assembly for use in sealing and EMI shielding applications, and particularly for aircraft related applications. More particularly, the present invention relates to an expanded, electrically-conductive mesh sheet positioned between adjacent surfaces of an external aircraft electronics component for use in shielding and protecting components which typically generate or receive electromagnetic radiation. Such external aircraft components include, for example, aircraft antennas, beacons, altimeters, radar and telemetry devices. The expanded mesh component is sealed from the environment with a chemically resistant elastomer having a deformable branched sealing edge disposed along the end portion of the gasket. The branched sealing edge extends above and below the gasket, and serves to protect the mesh sheet from fluids which could physically damage the mesh, or adversely affect its conductivity and shielding ability.
Electronic components are typically sources of electromagnetic (EM) radiation. Electronic components, for example, transmitters, transceivers, microcontrollers, microprocessors and the like, radiate a portion of the electric signals propagating through a device as EM radiation. The EM radiation generated in this way is sometimes referred to as EM noise. Higher operating frequency ranges of the electronic components leads to EM noise that primarily comprises radio frequency (RF) radiation. This RF radiation is normally referred to as RF noise. As may be used herein, EM noise and RF noise are referred to as EM radiation emitted from an electronic device. Moreover, EM noise and RF noise, unless otherwise stated, may be used interchangeably throughout the specification. EM radiation may also be emitted from electronic devices in close proximity to each other (EMI, or electromagnetic interference).
Many electronic devices are shielded to impede the emission of EM noise, or to prevent the device from receiving unwanted EM radiation. The shield is a physical barrier typically made of various conductive materials, for example, metal sheets, conductive plastic composites, conductive polymer sprays, metal filled epoxy pastes and the like. The shield absorbs EM radiation thereby impeding the emission of EM noise from an assembly of the electronic devices and the shield.
Gaskets are also used in EMI shielding applications where flexibility in addition to shielding effectiveness is required due to the particular application. Composite gaskets generally comprising a metal core material enclosed or encapsulated within a resilient polymeric material are known in the art. Such gaskets have sufficient structural integrity to be useful in sealing components in corrosive and high performance environments, such as for pressure vessels, automotive engines and aircraft. Examples of such gaskets are disclosed in U.S. Pat. No. 2,477,267 and U.S. Pat. No. 3,126,440. The disclosure of each of these patents is incorporated by reference herein.
U.S. Pat. No. 4,900,877 discloses a gasket for EMI shielding and environmental sealing. The gasket of this patent has a metallic electrically conductive deformable element adapted to be positioned in a gap between adjacent conductive surfaces. The gap is filled with a gel for sealing the space between the surfaces and for encapsulating the metallic structure to provide environmental protection against moisture and corrosion. The disclosure of this patent is also incorporated by reference herein.
The gaskets described in the aforementioned patents may not be acceptable for high performance applications, typically aircraft applications, where a variety of performance characteristics may be required in harsh working environments. For example, in addition to EMI shielding and sealing, electrical bonding of components and protection against corrosion may be a necessity.
Many of the commercial gaskets typically have electrical contact only at the edge portion of the gasket. Most such gaskets involve a woven flat wire mesh buried within the body of the elastomer not near the surface. When the gasket is cut to size, the wire mesh is exposed at the edge of the gasket and bent up near the surface. Since the electrical contact of these gaskets and the sealed components is at the edge portion of the gasket, a caulk must be applied at the edge of the gasket to protect the wire mesh from corrosion while maintaining the electrical bonding and EMI shielding. The application and curing of the caulk requires several hours of application and curing time, increasing down time of the equipment. When a gasket is replaced, the old caulk must first be removed, and the removal procedure can result in scratches to the protective coating of the equipment, requiring repainting of the surface, thereby expanding the scope and duration of the repair. Furthermore, most caulking compounds have a limited shelf life which can create inventory obsolescence and increase associated costs.
Additional problems are created when gaskets are used in external aircraft applications, such as for antennas and beacons. In these applications, the antenna is subject to deflection due to the wind shear. This can lead to the entry of fluids into the gasket assembly, including moisture, solvents, fuel, hydraulic fluids, de-icing fluids, etc. Many of these fluids are highly corrosive, and can quickly destroy the EMI shielding capabilities of the gasket. Typically, this problem is addressed through the use of polysulfide caulking materials applied to the periphery of the antenna after assembly. This approach usually requires significant downtime due to the curing time required for the polysulfide material.
It would therefore be highly advantageous to develop a commercial gasket with a greater deflection range than presently available, and which would also be able to provide environmental sealing between mating surfaces.
Accordingly, there is a perceived need for an improved deflectable EMI gasket design, particularly in aircraft applications, that would provide for improved sealing and EMI shielding.